The major growth factor signaling pathways in normal cells (e.g., PI3K and RAS) are also the ones that are most frequently genetically activated in cancer cells, leading to cell autonomous growth and proliferation. mTOR complex 1 (mTORC1) is a major driver of cell growth and is aberrantly activated in the majority of human cancers. This activation occurs through a network of upstream oncogenes and tumor suppressors that converge on a small G protein switch directly upstream of mTORC1. This switch involves the tuberous sclerosis complex (TSC) tumor suppressors, which form a protein complex (the TSC complex) that regulates a member of the Ras family of GTPases, called Rheb. How the TSC complex and Rheb integrate diverse upstream signals to properly control mTORC1, and how this regulation is perturbed in cancer cells continues to be a major focus of this grant, now entering its 3rd cycle. In the 4 years since the last renewal, we have made several new breakthroughs involving both the upstream regulation and downstream functions of the PI3K-TSC- mTOR network, resulting in 10 new research articles. For this renewal, we will build on and greatly expand from our most recent mechanistic findings regarding the TSC complex and its regulation of Rheb and mTORC1 (Dibble et al., 2012 Mol Cell; Menon et al., 2014 Cell). We found that the mechanism by which the PI3K-Akt pathway activates mTORC1 is through a phosphorylation-dependent dissociation of the TSC complex from Rheb at the lysosomal surface, where mTORC1 resides under conditions of sufficient nutrients. The stimulated release of the TSC complex from this location relieves its inhibition of Rheb, allowing Rheb to become GTP loaded and activate mTORC1. This novel spatial control mechanism explains not only how PI3K signaling activates mTORC1 but also how mTORC1 signaling integrates signals from nutrients and growth factors. In the next 5 years we will 1) define the molecular nature of the interactions and regulation between the TSC complex and Rheb, 2) reveal the mechanism by which Ras signaling stimulates mTORC1 through the TSC complex in both normal and cancer cells, and 3) determine whether differential regulation of the TSC complex and its localization dictate the sensitivity and resistance of cancer cells to targeted therapeutics, with a focus here on PI3K inhibitors in PIK3CA mutant breast cancers. This later hypothesis is based on a rapidly expanding literature providing evidence that sustained mTORC1 signaling following treatment with a targeted therapeutic against the major upstream oncogenic pathway in a given cancer is a major mechanism of both innate and acquired resistance to that drug. This has been observed in EGFR mutant lung cancer treated with EGFR inhibitors, B-Raf mutant melanoma treated with B-Raf or MEK inhibitors, and PIK3CA mutant breast cancer treated with PI3K inhibitors. Therefore, defining the molecular basis of mTORC1 activation by common oncogenic lesions and mechanisms leading to its resistance to targeted therapeutics in these settings is critical to predicting and improving clinical responses and is a major focus of the research under this proposal.